The invention relates to an arrangement for measuring the condition of a receiver antenna at a base station of a system, the arrangement comprising a receiver antenna positioned apart from other base station equipments, e.g., at a mast; an amplifying means positioned in connection with the antenna for amplifying an antenna signal; a means for generating a radio frequency measuring signal; a first directional coupler means for applying the measuring signal to an antenna line towards the antenna between the amplifying means and the antenna; a second directional coupler means for applying the measuring signal to the antenna line towards the receiver between the amplifying means and the antenna; switching means for switching the radio frequency measuring signal from the generating means alternately to the first and the second directional coupler means; means for measuring the strength of a measuring signal component sent towards the antenna and reflected back from the antenna and the strength of the measuring signal sent directly towards the receiver.
A substantial part of radio systems, for instance cellular radio telephone systems and base stations thereof, are receiver and transmitter antennas, the condition of which influences the quality of connections. A control of the condition of the antennas can be carried out e.g., by measuring the SWR (Standing Wave Ratio) of the antennas, i.e., electrical matching of the antennas to the remaining part of the receiver and transmitter system.
By means of present methods, the measurement is performed by supplying power through the antenna line to the receiver antenna and by measuring the power reflected back from the antenna along the antenna line by means of a broadband power meter. Due to the broadband power meter, the power used for the measurement must be high in order that the measurement will not be sensitive to interference, which means that signals received by the antenna do not disturb the measurement. Using high power causes intermodulation distortion in the receiver. These problems can be avoided by using a duplex filter separating from each other the measuring signal and the signals intended to be received. However, the duplex filter must be installed before the receiver parts, which weakens the sensitivity of the receiver, because the filter causes losses in the antenna signal.
A further solution is known, avoiding both above-mentioned problems by using a measuring frequency outside the receiver band and a narrow-band power measurement adapted to this frequency.
European Patent Application 0 261 828 discloses an analyzer of microwave range, measuring a signal supplied directly from a measuring source as well as a signal reflected from the microwave network to be analyzed, for a vector-based relative power measurement. At measurement of reflected power, a sample signal taken of the output power of a measuring generator is brought to a detector along a path different from the path of a sample signal taken of the power reflected from the microwave network to be analyzed. Thus, the relative measurement of the reference cited would be somewhat insensitive to variations in the output level of the measuring generator, but since the signals to be compared with each other use different paths to the detector, the measurement does not automatically consider non-idealities or changes in the properties of components present on signal paths. The analyzer must be calibrated for the measurement of reflected power by measuring a known microwave standard at first with the analyzer and by using thus-obtained calibration values for later measurements. For measuring the condition of an antenna, this would mean that the power supply of the antenna should be connected to some microwave standard instead of the antenna for the calibration.
This problem has been solved in the Finnish Patent Application 904085 in such a way that, in addition to the signal reflected from the antenna, also the strength of the measuring signal sent directly towards the receiver is measured separately. By this second measurement is obtained a reference value for the strength of the measuring signal, which value considers at the measuring moment the transmission power of a measuring signal transmitter and the properties of the components present on the signal path, such as amplifiers and branching elements, and with which value the strength of a measuring signal component reflected from the antenna is compared. By a relative measurement of this kind, it is possible to eliminate the influence of the components present in the measuring circuit and on the measuring signal path between the properties of individual components, or the influence of temporal changes in the properties of an individual component on the accuracy of measurement. Manual checkings and calibrations during installation and operation can also be reduced and simplified substantially or they can even be entirely avoided. The measuring signal is preferably a narrow-band signal, the frequency of which is outside the frequency band allocated for traffic, due to which the measurement does not disturb the actual radio traffic, but, on the other hand, the measurement is not performed at actually used frequencies either, and for this reason, an actual standing wave ratio SWR will not be obtained.
New digital TDMA-type (TDMA=Time Division Multiple Access) radio systems have time-division signalling including several, typically 8, time slots at one frequency. One TDMA system is the pan-European mobile telephone system GSM. The GSM Specification 12.11, 3.1.0 B 05 "Receiver Antenna Fault" states requirements for the control of the condition of the antenna.
With the antenna positioned at the mast and provided with a mast preamplifier, the condition of the antenna cannot be measured in a normal way, because the preamplifier is located on the antenna line between the feed point of the measuring signal and the antenna and the measuring signal should pass through this preamplifier in reverse direction. An attenuation of the amplifier in reverse direction is typically 40 dB. The accuracy of measurement is influenced by attenuations of even a few decibels, such as a receiver filter, which is also positioned at the mast. There is thus a need of providing the mast with a switching point for the measuring signal between the mast preamplifier and the antenna.